1. Field of the Invention
The present invention relates to a device for carrying out optical communication via optical fibers. In particular, the present invention relates to an optical module for carrying out bidirectional transmission and reception of optical signals over a single optical fiber.
2. Description of Related Art
The development of communication technologies in recent years has resulted in the implementation of single-fiber bidirectional optical modules used for carrying out transmission and reception of optical signals over a single optical fiber. Small-sized, high-speed, inexpensive optical modules are in particular demand for “Fiber-to-the-home” (FTTH), in which household-oriented data communication services are provided by running fiber-optics to individual homes.
FIG. 16 explains the configuration of a conventional bidirectional optical module. The optical module utilizes a waveguide type optical wavelength multiplexer/de-multiplexer, such as the one disclosed in JP H10-253848A, and has a configuration comprising: optical waveguide substrate 101 provided with transmitting optical waveguide 301 and transceiving optical waveguide 303 intersecting in a V-shape at one end; light-emitting element 201 provided at the other end of transmitting optical waveguide 301, i.e. at the tip of a prong of the V-shape; light-receiving element 202 provided at the intersection point of the V-shape; and optical filter 203 attached to optical waveguide substrate 101 between optical waveguide substrate 101 and light-receiving element 202, with optical fiber 200 secured to the tip of a prong of the V-shape of transmitting waveguide 301.
A received optical signal coming in from optical fiber 200 propagates along transceiving optical waveguide 303, passes through optical filter 203, impinges on light-receiving element 202, and is converted into an electric signal. On the other hand, a transmitted signal is converted into a transmitted optical signal by light-emitting element 201, coupled to transmitting optical waveguide 301, reflected by optical filter 203, caused to propagate along transceiving optical waveguide 303, and emitted into optical fiber 200. This makes it possible to perform transmission and reception of optical signals over a single optical fiber 200.
However, in the conventional optical module illustrated in FIG. 16, the transmitted optical signal, i.e. light that is emitted from light-emitting element 201 and is not coupled to transmitting optical waveguide 301, is scattered inside optical waveguide substrate 101 as stray light. When it is incident on light-receiving element 202, the stray light devolves into noise on top of the original received optical signal and impedes communication.
Furthermore, since the configuration of light-receiving element 202 is intended for direct reception of optical signals emitted from the end face of transceiving optical waveguide 303, it has to be provided separately from optical waveguide substrate 101, which creates problems in terms of increased cost and limitations on miniaturization.
FIG. 17 explains another conventional configuration, wherein the above-mentioned noise is reduced. In this conventional example, which is disclosed in JP 2005-091460A, slit 304 is formed obliquely to the normal to optical waveguide substrate 101 within transceiving optical waveguide 303, and optical filter 203 is inserted therein. Light-emitting element 201 is mounted above transceiving optical waveguide 303 such that transmitted optical signal 501 is coupled to transceiving optical waveguide 303 through optical filter 203. On the other hand, light-receiving element 202 is mounted so as to couple at the end of transceiving optical waveguide 303.
According to JP 2005-091460A, the fact that the light-receiving surface of light-receiving element 202 faces away from the light-emitting surface of light-emitting element 201 permits a reduction in noise. However, when the optical axis of the light-emitting element and the normal to the light-receiving surface of light-receiving element 202 intersect, especially when light-emitting element 201 and light-receiving element 202 are arranged in close proximity due to module miniaturization, a portion of transmitted optical signal 501 from light-emitting element 201 becomes incident on the light-receiving element directly and noise cannot be sufficiently suppressed.
Furthermore, the configuration of the module disclosed in JP 2005-091460A, in which transmitted optical signals 501 emitted from light-emitting element 201 are reflected by optical filter 203 and coupled to transceiving optical waveguide 303, places considerable limitations on the size and arrangement of light-emitting element 201, light-receiving element 202, and optical filter 203, and is inadequate in terms of productivity and miniaturization.